This PR is step 5 in the journey to solve issue #136139 and make the entire Flutter repo more readable.

(previous pull requests: #139048, #139882, #141591, #142279)

The current focus is on `packages/flutter/lib/src/material/`.  
The previous PR covered files in this directory starting with `a`, `b`, and `c`; this pull request is for `d` through `m`.